Note 113 — Chapter X · Boulton and the Steam Engine—Correspondence with Watt.

The following passage occurs in his letter:—“Suppose one piston up, and the vacuum made under it by the jet d’eau froid. That piston cannot yet descend, because the cock is not yet opened which admits the steam into its antagonist cylinder. Hence the two pistons are in equilibrio, being either of them pressed by the atmosphere. Then, I say, if the cock which admits the steam into the antagonist cylinder be opened gradually and not with a jerk, that the first mentioned [piston in the] cylinder will descend gradually and yet not less forcibly. Hence by the management of the steam cocks the motion may be accelerated, retarded, destroyed, revived, instantly and easily. And if this answers in practice as it does in theory, the machine cannot fail of success! Eureka!”